KDE Plasma 5.5: The Quintessential 2016 Review
Plasma 5.5 offers the first semi-usable Wayland session allowed directly from the log-in prompt. This should be considered a "technical preview" and is not yet ready for day-to-day use, unless you live solely in a web browser.
Wayland is the replacement for the obsolete and insecure X protocol using more modern methods of building desktop (and mobile) environments. Kwin, the KDE window manager, has been in the process of moving over to Wayland for some time now, and only in the past few months is the effort beginning to show visible results.
When you start into a Wayland session the only obvious visual clue you are in a Wayland session is the window decorations, which are the chunky blue defaults that come with Qt in lieu of the standard Breeze decorations. Some applications like Firefox do use the system borders, but I suspect this is more because they are using XWayland, as the KDE applications are using the Wayland blue borders.
There are several show-stopping Kwin-level issues for people wanting a usable environment; the mouse occasionally seems to "lose its place" and warp around (making resizing windows a challenge), cursors are regularly incorrect, windows can be incorrectly positioned, and there is some minor visual corruption in places. Frame-to-frame these issues cannot be captured in a screenshot, but in usage it's immediately apparent Plasma on Wayland will not be your daily driver.
I should note that taking screenshots with Spectacle doesn't work yet, but I have reconstructed a screenshot below.
Putting Wayland through its paces
Despite the buggy experience there are clear and obvious places where you can see KWin/Wayland shine. Using X11 you may not consciously notice moments of stutter, flickering, or tearing - but I never realised just how much my brain was suppressing these imperfections until a proper session with Wayland. The best way to describe it would be to say that Wayland removes the "mental fatigue" of constantly being exposed to many glitches.
During a simple (unscientific) stress test the worst issue was rapidly resizing a window and the window contents being one step behind the frame (despite Wayland using CSD in this version of Plasma Desktop), but it's worth mentioning X11 was choking in every way on the same test. That was the "worst" I could get by flinging windows around, resizing windows, using screen-edges, and rapidly opening/closing various popups, putting pressure anywhere I could think of... All of which suffer from a litany of now painfully obvious issues on X11.
I did cause session crashes while changing compositing settings, and I had to switch between GL versions and Xrender in an X11 session before I could look at the result in Wayland. Some KDE applications which I assume still rely on X have crashing issues, such as the system monitor which failed to start.
For the life of me I could not induce major visual glitches or flickers. For people looking at Wayland to boost performance, it isn't really giving more frames per second as much as it's giving better cand mroe complete frames, so there was a universal improvement in smoothness for various effects such as fading, wobbly windows, and expose. The general impression I got was that X11 will half-render, miss frames, or just glitch where Wayland consistently delivers on its “every frame is perfect” mantra. In terms of throughput or anything which could be benchmarked I didn't notice an appreciable difference, but my tests were anecdotal at best and I wouldn't want to test it as the Wayland session is still under active development.
Wayland Sessions in 2016 verdict:
At this point running a Plasma session in Wayland is very rough when viewed from the expectation of a user looking to do their daily grind, but on a technical level it's impressive to see the progress and you can see the light at the end of the tunnel.
While not strictly Wayland related, there is the issue of graphics interfaces. There are 3 main choices available with Kwin: XRender, GLX, and EGL. XRender is the most basic and using it severely limits what effects you can enable, EGL and GLX both offer the full gamut of eye-candy and visual polish. The issue I had is visual corruption only present with EGL such as noise being drawn instead of some shadows, and GLX being unavailable for Wayland. During my testing this meant Wayland sessions were either limited to basic XRender or a glitchy EGL.
Even discounting the EGL issue as limited to my particular machine, Wayland will not be your daily driver in Plasma 5.5 unless your needs are incredibly basic. It can serve stutter-free video and exceptional smoothness as a demonstration for interested users or developer-types but it's not yet ready for prime time, though it is extremely promising and making rapid progress.